1. Field of the Invention
The present invention relates to a flexure transducer element and a method of producing the same which is used for a semiconductor acceleration sensor having a both end supported beam structure and used for an automobile, an aircraft or a domestic electric appliance, and also relates to an acceleration sensor including such an element. For example, such a sensor can be used for sensing an acceleration by separately obtaining an X-axis component, Y-axis component and Z-axis component of the acceleration applied thereto with respect to an X-Y-Z coordinate rectangular system having the three axes.
2. Description of Background Information
The acceleration sensor as described above is disclosed in U.S. Pat. No. 5,485,749. The sensor is a piezoresistor-type acceleration sensor which converts a mechanical flexure (or a strain) of a member generated by an acceleration into an electric signal, and one example of such a sensor is shown in FIGS. 18 (a schematic perspective view) and 19 (a cross sectional view taken along a line A-A' in FIG. 18).
The acceleration sensor 500 includes a flexure transducer element 502 and a bottom cover 504. The flexure transducer element 502 includes a frame 506 and a sheet member 508. The frame 506 has an upper surface 510 and a lower surface 512 which is supported by a support member 514. The sheet member 508 includes flexible parts 515 and a center part 516 (a portion surrounded by the dash and dot lines in FIG. 18). The flexible part 515 extends outward from the center part 516 and integrally connects with an inner edge 518 of the frame 502. A weight 520 connects with the center part 516 of the sheet member 508 below the center part 516.
An inward side surface 524 of the support member 514 is facing to an outward side surface 526 of the weight 520 through a first space 528. Further, a second space 530 is present between the flexible parts 515 and the weight 520, and connects with the first space 528. In addition, there is a third space 532 which is surrounded by the frame 506 and the flexible parts 515. The flexible parts 515 include a plurality of piezoresistors 534 and wirings (not shown) connected thereto on their surfaces.
The bottom cover 504 includes a periphery part 541 which defines a recess part 540 corresponding to and surrounding the weight 520, and the support member 514 is bonded to the periphery part of the bottom cover 504 by an appropriate means such as anodic bonding. The bottom cover 504 functions as a stopper which prevents the sheet member 508 from being broken due to over-displacement of the weight when an excessive acceleration is applied.
When the acceleration sensor 500 as described above includes the plurality of the piezoresistors 534, it can be used as the acceleration sensor which detects the acceleration by obtaining separately the X, Y and Z axis components of the acceleration applied to the sensor with respect to the X-Y-Z three axis regular coordinate defined by the X, Y and Z axes which regularly intersecting with one another (the X axis and the Y axis extend over the upper surface defined by the sheet member 508 and the frame 506).
Interconnections between the frame 506 and the sheet member 508 as well as between the sheet member 508 and the weight 520 are such that when the acceleration is applied to the sensor 500, concretely to the element 502, at least a portion of the flexible part 515 which portion has the piezoresistor 534 is elastically deformed by the displacement of the weight 520 relative to the frame 506 (it is noted that the center part 516 of the sheet member 508 which is connected to the neck part 522 is substantially not deformed), and thereby a resistance change of the piezoresistor 534 is converted to an electric signal. By detecting the signal, the acceleration applied to the sensor is determined.
The production of the acceleration sensor as described above can be carried out based on a method disclosed in the U.S. Pat. No. 5,485,749, and concretely carried out as follows using a production sequence as shown in FIG. 20 which shows schematic cross sectional views similar to FIG. 19:
FIG. 20(a): First, a silicone nitride films 602 and 604 are formed on the both surfaces of a first silicon substrate 600 from which the support member 514 and the weight 520 are to be formed.
FIG. 20(b): Then, an opening 606 is formed by removing a portion of the silicon nitride film 602 which corresponds to the second space 530, and an opening 608 is formed by removing a portion of the silicon nitride film 604 which corresponds to the first space 528.
FIG. 20(c): By digging from the openings 606 and 608 to form recess parts 610 and 612 respectively, and then remaining silicon film 602 is removed so that one surface of the first silicon substrate 600 is exposed, on which a second silicon substrate 616 is laminated so that a portion of the recess part 610 is formed into the second space 530 and the rest part is formed into the neck part 522 of the weight and the upper surface of the support member 514.
FIG. 20(d): In order that the flexible part 515 is deformed upon the application of the predetermined acceleration when finally completed as the sensor, the second silicon substrate 616 is thinned to a thickness (t) by grinding or etching, whereby the second silicon substrate is formed into the sheet member 508 and the frame 506.
FIG. 20(e): Then, the piezoresistors 618 are formed on the sheet member 508 of the thinned second silicon substrate 616 using diffusion of an impurity of which conductivity type is different from that of the second silicon substrate 616.
FIG. 20(f): Then, after wirings (not shown) connected to the piezoresistors 618 are formed, a first space 528 reaching the third space 530 is formed by anisotropic etching from the recess part 612 so that the weight 520 is connected to and supported integrally by the center part 516 of the second silicon substrate 616 through the neck part 522.
Finally, the predetermined portion of the second silicon substrate 616 is etched so that the third space 532 (not shown) is formed, whereby the flexure transducer element 502 is obtained. It is noted that the silicon nitride film 604 on the bottom surface of the first silicon substrate may be optionally removed.
The element 502 thus obtained is bonded to a bottom cover 504 (not shown in FIG. 20), which results in the piezoresistor-type acceleration sensor.
Alternatively, the following method is also known: the second space 530 is not formed directly from the substrate, but a portion which corresponds to the second space is once formed as a sacrificial layer of a polysilicon, and then the sacrificial layer is removed by supplying an etchant through the first space 528 after the first space 528 has been formed (see Japanese Patent Kokai Publication No. 7-234242 and its counterpart foreign patent applications if any and U.S. Pat. No. 5,395,802).
In such an acceleration sensor, the acceleration to be detected is converted to a flexure of the flexible part as at least a portion of the sheet member, so that the resistance of the piezoresitor formed on the flexible part is changed by means of the flexure, whereby finally the acceleration is converted to the electric signal.
Therefore, the sensitivity of the semiconductor acceleration sensor is controlled by particularly the thickness of the flexible part of the sheet member which is elastically deformed (or flexed). That is, when the flexible part becomes thicker, the sensitivity becomes worse, and the sensitivity is affected by scattering of the thickness of the flexible part. Thus, the uniform and precise control of the thickness of the sheet member is important in the production process of the semiconductor acceleration sensor.
As another type of the sensor, an electrostatic capacitance-type sensor is also known, and it is disclosed in for example Japanese Patent Kokai Publication No. 5-26754 and its counterpart foreign patent applications (if any) and Europe Patent Publication No. 0 461 265. Operation mechanism of such a sensor is similar to the piezoresistor-type sensor in that it is based on the mechanical flexure formed by the application of the acceleration. However, it is different from the piezoresistor-type sensor in that the flexure is converted to relative displacement between two opposing members, and the displacement changes the electrostatic capacitance between electrodes provided on the members, which is utilized in the electrostatic capacitance-type sensor. Thus, in the electrostatic capacitance-type sensor, the electrodes are provided on the member which is displaced and the member which is not displaced upon the application of the acceleration sensor so that these electrodes are opposing to each other.
Such an acceleration sensor is shown in FIGS. 21 (a schematic partially cut-away perspective view) and 22 (a schematic cross sectional view taken along a diagonal C-C' in FIG. 21). While the above flexure transducer element 502 includes the piezoresistors 534, the flexure transducer 702 of the acceleration sensor 700 includes in place of the piezoresistors, the electrode 734 on the upper surface of the weight 520 and the wiring 736 connected thereto, and the wiring is provided on the sheet member through the depressed corner 738 of the third space, The other features are substantially the same as those of the above piezoresistor-type flexure transducer element 502 shown in FIGS. 18 and 19.
It is noted that the flexure element 702 of the electrostatic capacitance-type is used with the top cover 740 (not shown in FIG. 21) which is located on the element. The top cover 740 prevents excessive displacement of the weight, whereby prevents break of the flexible parts, and includes on its inside, a recess part which corresponds to at least the sheet member and preferably an upper surface of the element except the frame. This kind of top cover is combined with the element for the piezoresitor-type acceleration sensor or the electrostatic capacitance-type acceleration sensor, provided that in the latter type sensor, the top cover includes an electrode as described below. The top cover 740 includes the electrode 742 which faces to the electrode 734 when the cover is placed on the element 702. In such an acceleration sensor, when the acceleration to be detected is applied to the sensor, the weight 520 is displaced relatively to the support member 514 and thus the cover 740 arranged thereon since the weight 520 is connected to the sheet member 508 including the flexible parts 515. As a result, a distance between the electrode 734 on the weight and the electrode 742 opposing thereto on the cover is changed, whereby the acceleration can be sensed using an electrostatic capacitance change between the electrodes which is caused by the distance change.
Also in this acceleration sensor of the electrostatic capacitance-type, when the thickness of the flexible part 515 is thinner, and also when the length of the flexible part is longer in the case of the flexible part being in the elongated form, the flexible part is more likely to be deformed even with a smaller acceleration, which improves the sensitivity of the acceleration sensor. Also, when the thickness of the flexible part has scattering, scattering of the sensitivity occurs.
Therefore, in any type of the acceleration sensor, it is desirable that the thickness of the flexible part is properly controlled so that the semiconductor acceleration sensor or the flexure transducer element is provided which includes the flexible part having less scattering in their thickness. Thus, it is important to precisely control the uniform thickness of the flexible parts in the production method of the transducer element. Further, when the flexible part is in the elongated form, it is preferable that its length can be longer.
In the production method of the prior art for the semiconductor acceleration sensor as described above provides such a sensor having a both end supported beam structure in which the weight is connected to the center part of the sheet member, the flexible parts of the sheet member are connected to the frame, and the frame is supported by the support member.
In this production method, since the thickness scattering of the second silicon substrate is large in the step of thinning the second silicon substrate up to the predetermined sheet form thickness (t) after laminating the second silicon substrate 616 onto the first silicon substrate 600, it is difficult to control the thickness of the flexible part 515 uniformly. Further, lamination of the silicon substrates is complicated and two pieces of the silicon substrates are required, which increases the production cost.